When a telecommunications satellite is launched into its operational orbit, a number of tests should be carried out to make sure that all the functionalities are validated before putting it in an operational state.
It also happens that a number of tests are performed during the operational phase of the satellite. These tests should preferentially be performed without service outages in particular with regard to operators exploiting broadcasting pathways passing through the satellite.
Telecommunications satellites also ensure a repeater function, that is they retransmit in a predefined zone a signal sent to the satellite for example for a TV broadcasting application.
The satellite comprises a set of equipment forming for example a signal controlling, processing, amplifying, addressing and broadcasting system.
A particularly sensitive equipment is the amplification chain which can comprise one or more amplification channel(s) corresponding to a demultiplication of the satellite capabilities. These channels are called “paths” when they allow signals arriving at the input ports of the MPA to be decomposed and recomposed. The MPA comprises components in particular of a so-called “Butler” matrix allowing components divided by an input block to be amplified and phase shifted and to be recomposed by an output block at the output ports. Each transmission pathway is allocated to an input port and an output port. The signals of a pathway can thus be, at the input of the MPA, decomposed in different paths and recomposed after amplification at the output to be conveyed to a transmitting antenna of said pathway.
Currently, different repeater solutions integrate a multi-port amplifier on-board a telecommunications satellite. An amplification path of a satellite repeater generally comprises a travelling wave tube amplifier. This is a wide band amplifier with a very low background noise. Generally, a telecommunications satellite includes an MPA enabling several paths to be processed enabling the amplification of components of signals from different pathways of a certain bandwidth of frequencies. Each pathway can be “rented” or used by broadcasters or operators. It is thus desirable that each path amplifying and phase shifting the components of an input signal of the MPA be calibrated so as to provide an orthogonality function between the components divided in the different paths of the MPA.
The multi-port amplifiers are particularly used for missions requiring the coverage of a plurality of places, each designating an earth station. The satellite allows the transmission of a plurality of downlink beams generated from a power management adapted to each pathway and a management of assignment of input and output ports and conveying signals to the satellite antennas. These solutions allow flexibility regarding the allocation of the power required on each output port of a multi-port amplifier.
The multi-port amplifiers are commonly called “MPA” in the state of the art.
Generally, an MPA comprises for each of its pathways a travelling wave tube amplifier (TWTA). It is reminded that the TWTA is a vacuum tube used in microwaves to make low, medium or high power amplifiers. It enables wideband and very low background noise amplifiers to be made. It is particularly suitable for the amplifiers of communication satellites.
Further, an MPA comprises a network for forming beams enabling one or more beam(s) emitted with a given power, phase and direction to be developed. An example of such a commonly used network is a Butler matrix with N inputs and N outputs and a specific configuration of paths comprising amplifiers and phase shifters and allowing the achievement of a configuration of phase shifting and amplifying the components from the input signals of the MPA which are divided to obtain one or more desired output signal(s) of the MPA.
In a nominal operation, if a pathway is defined between the input port N° 1 and the output port N° 1, then a signal conveyed to the input port N° 1 is only present on the output port N° 1. The feature of an MPA is that the signals of a same pathway are phase shifted in each of the paths according to a plan defining the phase shifts from one path to the other. In each path, the phase shifted signals are amplified by a TWTA type amplifier. In practice, a chosen design of a network for forming and recomposing beams can be ensured by a Butler matrix. The latter provides a so-called “unate function”. The unate function contributes to the formation of a recomposed beam at an output of the matrix and potentially to the formation of N recomposed beams at each of the outputs.
One benefit is that each active TWTA contributes to the amplification of signals of different pathways. If a plurality of signals, separated in frequencies, are routed to the different inputs of a matrix, each TWTA amplifies the signals of each pathway.
When an MPA is well calibrated, the signals conveyed to a first input port of a pathway are only present at the output of a first output port. A problem occurs when the calibration between the different paths of the MPA is not properly performed. Indeed, a phase and/or amplitude drift of the components between the different paths can result in making resultants of components of signals at the output of the MPA non-null whereas a configuration of the MPA provides that the latter should be substantially null at the output of the other output ports. This results from a functionality of a Butler matrix as detailed in FIG. 2 which enables components of signals in opposite phase at the output of the MPA to be cancelled. This problem is generally called an unbalance of the MPA.
An unbalance of an MPA can cause different consequences among which:                a reduction in the power of a main signal at the output of a port of the matrix because the summation of the in-phase components are slightly phase shifted.        the reduction in the power of a main signal at the output of a port of the matrix caused by a gain difference between different paths of an MPA;        the creation of significant signal levels, called leaks, at the outputs of some ports because of a phase shift drift which does not make a resultant of components summed among each other in opposite phases substantially null any longer.        
A commonly accepted limit is that the power of the leaks should be in the order of a power 25 dB lower than the power of the main signal on the same pathway.
Today, a way to solve the problem of the unbalance of an MPA is to parameterize the phases and amplitudes of each TWTA pathway. But a problem related to decalibration or unbalance of the pathways remains present with aging of the TWTA modules or even aging of the inputs and outputs of a Butler matrix or other components.
Another drawback of this solution is that when a TWTA is failing and a second TWTA is chosen to replace it, it is not previously calibrated in amplitude and phase relative to the other TWTA of the MPA.
It is thus desirable to measure the amplitude and phase unbalances of a path of the MPA relative to the other amplification paths to dynamically correct the calibration. Finally, one of the major problems encountered is to carry out measurements and a recalibration without disturbing current communications of the MPA on each of the other paths.
A first known solution consists of measurements performed on-board the satellite and transmitted to the ground in a so-called “open loop” architecture. In this solution, the calibration of a path of the MPA is performed by carrying out measurements on the signals of outputs of the matrix. On-board RF detectors are then used which are connected by means of one or more coupler(s) to ports non-used by the transmissions relayed in the satellite. RF detectors enable power levels of the signals at the output of the non-used ports to be measured. The measurements are performed on each port independently of each other. The measurements are then transmitted to a ground station by means of the telemetry link. For example, an increase in the RF level measured at the output of one of the ports relative to the level injected reflects a degradation in the calibration of the MPA.
A problem of this solution is that it depends on the operational configuration chosen in particular according to the choice of the input levels of the test signals transmitted. The measurements sent to a ground station can be unusable or unexploitable to deduce therefrom a recalibration to be performed. The major drawback of this solution is the test dependence on the chosen operational configuration.
A second solution consists of measurements performed on-board the satellite and transmitted to the ground in a so-called “closed loop” architecture. One or more test signal(s) is (are) generated on-board by means of a DSP and is/are injected in one or more input ports of the matrix. The signals at the output of the output ports are collected via one or more calibrated coupler(s). The input signals are also injected by means of a calibrated coupler on the input port of the matrix. The outputs of the matrix can be looped back to the DSP such that the latter adjusts the phase shifts and amplitude differences of the signal generated at the input of the matrix.
The drawback of such a system is to be expensive. Further, it is desirable to take a complex architecture on-board the satellite in particular by providing a ASP especially designed to perform this calibration test. Hence, the components add further weight on-board the satellite.
A third solution consists in directly receiving the signals transmitted on the channels to be tested/calibrated in a plurality of ground stations. The frequency band and directivity of the antennas are then chosen so as to allow these transmissions from the satellite. An exemplary implementation consists for example in choosing a main earth station on which a main signal is transmitted from the satellite. The antenna and the corresponding pathway of the satellite are configured to transmit this main signal on a predefined station. Besides, a plurality of places geographically distinct from the main station each corresponding to the transmission on a pathway of the MPA are chosen. This method consists in measuring losses of lines of the primary pathway on at least one other pathway by measuring in each place the received power corresponding to the signals of the primary pathway. For this, the RF levels on each of the pathways are measured on the ground for each place. After the signals are reconstituted, within the antenna attenuations, it is possible to deduce the line losses caused by the MPA. Comparisons of the signals received enable losses caused by an unbalance in one or more pathways of the MPA to be deduced and isolated.
A major drawback of this solution is that it disturbs the current telecommunications of the satellite when it is in an operational configuration.
In conclusion, today solutions for measuring an unbalance of the MPA and correcting these unbalances are either expensive, or difficult to implement and disturbing for current operational communications when the satellite is operational.